1. Field of the Invention
The present invention relates to an electrical switch for electric tools having an electric motor operated by a DC voltage.
2. Description of Prior Art
Electrical switches such as these are used in particular as mains switches for electrical handheld tools. Particularly in the case of electric motors which are operated by means of a DC voltage, for example 125 volts, heavy currents can flow, for example a direct current of 130 Amperes or more, so that an arc occurs between the contacts on switching off. The occurrence of arcs can lead to the switch being destroyed, and this must be prevented, for safety reasons, in electrical tools.
DE 197 26 402 A1 discloses an electrical switch for heavy currents, which is provided with means for quenching an arc. This switch has a contact system which comprises a stationary contact and a switching contact. An operating element acts on the switching contact in order to initiate a process for switching the contact system between an off-position and an on-position. The means, which is associated with the contact system and is in the form of a shield, for quenching any arc which occurs between the stationary contact and a switching contact during the switching process is movably coupled to the switching contact. In consequence, the shield is moved into the area located between the stationary contact and the switching contact during the process of switching the switching contact from the on-position to the off-position, and is moved out of the area located between the stationary contact and the switching contact during the process of switching the contact from the off-position to the on-position.
When switching the contact system on, the shield is moved downward, so that the shield is located underneath the stationary contact when in the on-position. A certain amount of space is therefore required there in the housing of the switch, for which purpose the housing has a bulge in this area. In some applications, particularly when there is a shortage of space available in the handle of the electrical appliance, this bulging in the switch housing may cause interference. Furthermore, the coupling between the switching content and the shield involves a complex design, resulting in the shield moving in a complicated way when switching between the off-position and the on-position. This necessitates high precision for production of the parts for the shield and, in the end, this increases the production costs for the switch.
The invention is based on the object of developing the shield further in such a manner that the switch housing can be further reduced in size.
For a switch of this generic type, this object is achieved by an electrical switch for electrical tools having an electric motor operated by a DC voltage, comprising: a contact system including a stationary contact and a switching contact; an operating element which acts on the switching contact to initiate a process for switching the contact system between an on-position and an off-position; a shield that quenches an arc that occurs between the stationary contact and the switching contact during the switching process; a movable slide being operatively connected to the operating element and having the switching contact arranged thereon; wherein, during the process of switching the switching contact from the on-position to the off-position, the shield is moved to an area located between the stationary contact and the switching contact and, during the process of switching the switching contact from the off-position to the on-position, the shield is moved from the area located between the stationary contact and the switching contact; and wherein one end of the shield is moveably associated to the slide and the other end of the shield operates as a shielding element for the switching contact, when the shield is moved to the area between the stationary contact and the switching contact.
In one preferred development, the shield is in the form of a two-armed lever element. The first lever arm of the lever element is articulated on the slide, and the second lever arm is associated with the switching contact, as the actual shielding element. The two-armed lever element may be approximately C-shaped or sickle-shaped. The switch housing may comprise a housing lower part for holding the slide, the contact system and the like, and the housing upper part in the nature of a cover. In this case, the rotation part of the two-armed lever, may be mounted in the interior of the housing, in particular on the housing lower part.
In order to articulate the shield on the slide, a guide link is located in the slide and/or in an attachment arranged on the slide. A bolt which is fitted to the first lever on the shield engages in the guide link in such a manner that the shield can be moved positively with the slide. The guide link may comprise a first guide surface, facing the switching contact, and a second guide surface, which is adjacent to it and faces away from the switching contact. The first guide surface is in the form of an inclined plane, which runs obliquely with respect to the movement direction of the slide, while the second guide surface runs approximately in the direction of movement of the slide. Overall, the guide link is approximately boomerang-shaped. This type of articulation ensures precise positive guidance between the slide and the shield.
The housing lower part has a recess for holding at least one part of the second lever arm of the shield when the contact system is in the on-position. Furthermore, the recess is also used for holding part of the first lever arm of the shield when the contact system is in the off-position. At least one side surface can be arranged on the second lever arm, which acts as the shielding element, in such a manner that the shielding element forms a type of cap. The cap then largely covers that surface of the switching contact which faces the stationary contact in the off-position. This refinement reliably prevents the arc from escaping sideways.
In one preferred refinement, the switch according to the invention has a contact system with two stationary contacts and one switching contact which is in the form of a bridging contact and is provided with two contact surfaces. The shield comprises two lever elements which are arranged alongside one another and are associated with the respective contact surface. The two lever arms of the lever element of the shield can be connected to one another at their rotation point via a common shaft. The shaft expediently has ends which project at the rotation point of the lever element, with the two ends of the shaft each being mounted in a holder in the housing lower part such that they can rotate. Furthermore, two mutually opposite attachments can be arranged on the slide, each having a guide link, in such a manner that the bolt on the first lever arm of each lever element engages in the guide link.
The advantages achieved by the invention are, in particular, that the switch is reliably protected against its contact system being destroyed by the influence of arcs. Any arc which may occur is quenched within a short time, thus effectively preventing erosion of the contacts in the contact system. The life of the switch is thus considerably increased.
The electrical switch is particularly suitable for heavy currents and can thus be used in high-powered electrical tools. In particular, the switch can be used for electric motors which are operated with DC voltage, where, in contrast to the situation at the zero crossing of an AC voltage, the arc is not automatically quenched. The switch can also be used as a brake switch for electrical tools that use short-circuit breaking, where particularly large direct currents occur on switching off, due to the electric motor being short-circuited and acting as a generator.